Over the years, prosthetic hand replacements have remained substantially unchanged, generally comprising a split hook used to grasp, and/or otherwise handle a work piece or a mechanical hand which includes a two-fingered gripping arrangement.
Likewise, the mounting unit for such artificial devices generally comprises a residual limb socket having a cup at one end. A wrist unit is secured in the cup end. The wrist unit serves as a mounting for the artificial hand or hook.
Residual limb sockets currently in use are comprised of fiberglass or synthetic fiber reinforced plastic or a similar material. These are custom made and very costly. The socket is matched in size and configuration to the residual limb to receive the prosthetic. The match need be so close that a thin fabric stockinette is all that is required between the residual limb and the shell. Naturally the cost of fabrication is extremely high. The closed end of the residual limb socket or shell terminates in a metal mounting member which includes a threaded hole which serves to receive the hand or hook. The hand or hook is kept in position by conventional mechanical means which involve the use of tools which are used to tighten the hand or hook in a firmly fixed angular position in a classic metal to metal tightened joint.
Such prior art devices suffer from numerous inadequacies. More particularly, such prior art devices, due to the fact that their structure is largely made of plastic, will tend to fatigue and break under heavy use. Fatigue also results in a change in the residual limb socket size. As the residual limb socket must be closely matched to the residual limb receiving the prosthetic, fatigue of the plastic residual limb socket results in changing the inner dimension of the residual limb socket, causing a poor fit and in some cases trauma, thus necessitating replacement of even an unbroken residual limb socket or shell. It is noted that while the prior art has stressed the use of various fibers to reinforce the plastic, such reinforcement does not solve the problem of residual limb socket or shell fatigue.
In addition, under use, the cup defined in the closed end of the residual limb socket or shell will tend to break, resulting in the metal mounting member losing its support. Thus, prior art prosthetic devices do not work well for even short periods of time under relatively heavy use, such as that which might be needed by a farmer, auto mechanic, construction worker, or the like.
Finally, even if, at least initially, the shell does not break, heavy use results in severe discomfort due to the relatively hard interface between the residual limb and the socket.
In principal, one could consider several approaches toward solving the above problems. For example, the plastic could be replaced with relatively robust materials, such as steel, but steel would present the problem of extra weight as well as constructional complications insofar as the prosthetic must be custom fitted to each user. Thus, the easy workability of, for example, fiberglass or synthetic fiber reinforced materials, renders them more desirable than metals, because of the object of custom fabrication adapted to the particular user. In addition, a steel prosthetic residual limb socket or shell would be relatively heavy and will change the balance of the device and thus be impractical to use.
While one could, perhaps, address the problem by increasing the diameter of the residual limb socket or shell at its closed end, the structure would be relatively cumbersome to use and unnatural in appearance, although an increased cup size may provide some additional strength. Nevertheless, the amount of improvement which could be achieved by such an artificially large cup size would not be significant.
Still another problem with prior art devices is the fact that tools and highly skilled technicians are needed to fabricate, install and adjust the prosthetic unit.
Moreover, once adjusted, the prosthetic wrist unit of conventional design will not, unless under significant force, easily, adapt to movement outside its initial setting. Thus, for example, if the angular position of a hook is set in the conventional way and the individual is driving a car and a sudden movement necessary to avoid an accident is made, the prosthetic unit will not accommodate that movement well and may result in locking the prosthetic device in the steering wheel and a loss of control.